TW201530882A - Base layer for battery electrodes, collector using same, electrode and lithium ion secondary battery - Google Patents

Abstract

Provided are: a base layer that is capable of suppressing decrease of adhesion between a collector and an electrode layer; a collector with a base layer, which is obtained by forming the base layer on a collector; an electrode which is obtained by additionally coating an electrode layer on the collector; and a lithium ion secondary battery which is provided with the electrode. This base layer is provided between a collector and an electrode layer of an electrode of a secondary battery, and contains conductive materials and an amorphous polyester resin having a bisphenol A skeleton in the main chain, said amorphous polyester resin serving as a binder for binding the conductive materials.

Description

Base layer for battery electrode, current collector using the same, electrode, and lithium ion secondary battery

The present invention relates to a substrate layer of a lithium ion secondary battery and an electrode having the substrate layer. More specifically, the present invention relates to a base layer between a current collector and an electrode layer, a current collector having a base layer on which the base layer is formed on the current collector, and further an electrode layer coated on the surface thereof. An electrode, and a lithium ion secondary battery using the electrode.

The electrode member for a lithium ion secondary battery is used for, for example, a battery (including a capacitor) used in an electric vehicle, a fuel cell vehicle, a hybrid electric vehicle, a household electrical storage device, a power tool, a train, a small portable device, or the like. In particular, lithium ion secondary batteries have been mounted on automobiles and the like in recent years, and their development has been remarkable.

The members constituting the lithium ion secondary battery can be roughly classified into a positive electrode, a negative electrode, a separator, and an electrolytic solution. Among them, the electrode is composed of a material such as a current collector, an active material, an adhesive, and a conductive auxiliary material, and the performance of the battery as a whole is greatly affected.

The electrode is formed by dispersing and mixing a material containing an active material, an adhesive, a conductive material, and the like with a solvent (as a function that also functions as a dispersion medium) on a current collector using a metal foil or the like. Slurry. After that, usually by being part of the coater It is manufactured by drying and winding in an oven which is assembled. Thereafter, slitting or pressurization is performed as necessary.

However, when the electrode layer formed on the current collector of a metal foil or the like is repeatedly charged and discharged, the interface between the current collector and the electrode layer is deteriorated to increase the electric resistance and reduce the discharge capacity. Therefore, the charge and discharge cycle life is insufficient. Further, there is a problem that the powder of the electrode layer that has fallen off from the current collector is a cause of a short circuit or the like.

It is considered that due to the doping and dedoping of lithium ions accompanying charge and discharge, local shear forces are generated at the interface between the electrode layer and the current collector due to repeated expansion and contraction of the active material, and the shear force is used. The interface adhesion between the electric body and the electrode layer is deteriorated, and peeling of the current collector and the electrode layer occurs.

Patent Document 1 discloses an electrode layer comprising carbon black, a polymer compound containing a fluorine-based polymer compound, and a thermosetting crosslinking agent, and a base layer which has been subjected to a heat curing treatment is interposed therebetween. On the collector. Further, Patent Document 2 discloses an electrode layer characterized in that it contains carbon black and a polymer compound which can be cured by irradiation with radiation, and the base layer which has been subjected to radiation hardening treatment is interposed in the middle to be formed on the current collector. . However, in the methods disclosed in Patent Documents 1 and 2, when the underlayer is formed, the use of a curing agent makes the hardening step necessary and the productivity is lowered.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 7-201362

[Patent Document 2] Japanese Patent Laid-Open No. Hei 7-201363

The present invention has been made based on the fact that the substrate layer capable of suppressing the decrease in the adhesion between the current collector and the electrode layer and the substrate layer having the base layer formed on the current collector are provided. A body, an electrode further coated with an electrode layer on the surface of the current collector, and a lithium ion secondary battery provided with the electrode.

The present inventors have found that by using an amorphous polyester resin having a bisphenol A skeleton in the main chain as an adhesive contained in the undercoat layer, the above object can be attained without using a hardener.

That is, one aspect of the present invention is a base layer which is a base layer provided between a current collector and an electrode layer constituting an electrode of a secondary battery; and a conductive material and a conductive material for bonding to each other An amorphous polyester resin having a bisphenol A skeleton in the main chain with an adhesive.

Further, the conductive material may be a carbon material.

Further, the carbon material may also be acetylene black.

Further, the number average molecular weight of the amorphous polyester resin having a bisphenol A skeleton in the main chain may be in the range of 14,000 or more and 22,000 or less.

Another aspect of the present invention is a current collector with a base layer formed with a base layer as described above on a current collector.

Further, the current collector may be an aluminum foil.

Another aspect of the present invention is an electrode which is the above An electrode layer is further laminated on the base layer of the current collector with the base layer.

Further, the electrode layer may contain an active material for a positive electrode.

Another aspect of the present invention provides a lithium ion secondary battery comprising the above electrode.

According to the present invention, it is possible to provide a base layer capable of suppressing a decrease in the adhesion between the current collector and the electrode layer, and a current collection layer having a base layer formed on the current collector in a step suitable for mass production. A body, an electrode further coated with an electrode layer on the surface of the current collector, and a lithium ion secondary battery provided with the electrode.

Fig. 1 is a graph showing the results of a charge and discharge cycle evaluation test.

[Formation of the Invention]

Hereinafter, an embodiment of the present invention will be described in further detail.

The underlayer of the present embodiment is characterized in that it is provided between a current collector constituting an electrode of a lithium ion secondary battery and an active material layer; the base layer is formed by an adhesive for adhering the conductive materials to each other. The adhesive is an amorphous polyester resin having a bisphenol A skeleton in the main chain. Here, the bisphenol A skeleton refers to a polymer structure containing a molecular structure derived from bisphenol A in a main chain structural unit (repeating structure) of a polymer. Hereinafter, the base layer will be described.

(adhesive)

First, the adhesive of the present embodiment will be described. In general, polyvinylidene fluoride, polytetrafluoroethylene, EPDM (ethylene-propylene-diene rubber), SBR (styrene/butadiene rubber), NBR (nitrile rubber), fluororubber, etc. can be used as the adhesive. A material that is stable and stable in physical properties, but must be used in a solvent dissolved in a slurry but not dissolved or swelled in the electrolyte of the battery. Therefore, as the adhesive of the present embodiment, an amorphous polyester resin having a bisphenol A skeleton in the main chain is used. Since a solvent such as dimethyl carbonate, diethyl carbonate, ethyl carbonate, propyl carbonate or butyl carbonate is used as a solvent in the electrolyte, the adhesive does not dissolve or swell in the solvent. It is a necessary condition in its mixture. Here, as a result of the experiment by the inventors, most of the amorphous polyester resins having no bisphenol A skeleton in the main chain are dissolved or swelled in the carbonate; conversely, there is no bisphenol A skeleton in the main chain. The shaped polyester resin has superior acceptability to carbonate. Therefore, when an amorphous polyester resin having a bisphenol A skeleton in the main chain is used as an adhesive, when a usual carbon material is used as a conductive material to be described later, an electrode having excellent cycle characteristics can be obtained. The remarkable effects of the present invention can be obtained. Further, the number average molecular weight of the amorphous polyester resin having a bisphenol A skeleton in the main chain is preferably in the range of 14,000 or more and 22,000 or less. The amorphous polyester resin having a bisphenol A skeleton in the main chain has a number average molecular weight of less than 14,000, and will swell once immersed in the carbonate. Further, in the case where the number average molecular weight of the amorphous polyester resin having a bisphenol A skeleton in the main chain is larger than 22,000, it becomes difficult to dissolve in the solvent of the slurry.

(electrically conductive material)

Next, the conductive material will be described. As the conductive material, a carbon material such as Ketjen black, acetylene black, carbon black, graphite, carbon nanotubes, or amorphous carbon is preferably used. Among these, acetylene black is a structure in which carbon particles having a very small particle diameter of 50 nm to 200 nm are connected in a chain shape, and acetylene black is preferably used in order to have a superior shape for forming a conductive path. Further, in the description of the present invention, although it is labeled as "conductive material", it is the same as that indicated as "conductive agent".

(Production of base slurry)

Next, a method of producing the base slurry will be described. The base slurry system can be obtained by mixing an adhesive, a conductive material, and a solvent. Further, the solvent also has a function as a dispersion medium for dispersing the conductive agent. The mixing method is not particularly limited, and a general agitating mixer such as a rotary type or a planetary type can be used. It is necessary to select a solvent that dissolves the adhesive. As the solvent, for example, when an amorphous polyester resin having a bisphenol A skeleton in the main chain is used as the adhesive of the present embodiment, toluene is preferably used. However, a mixture of toluene and MEK (methyl ethyl ketone) is more preferably used from the viewpoint of drying speed at the time of coating or the like.

First, the adhesive is poured into the above solvent, and it is sufficiently dissolved using a stirring mixer. Next, a conductive material is placed and further stirred and mixed. Although the time for stirring and mixing is not particularly limited, since the conductive material must be sufficiently uniformly dispersed, the base slurry can be usually obtained by performing for several tens of minutes to several hours.

(formation of the basal layer)

Next, a method of forming the underlayer will be described. By going up The obtained base slurry is applied onto a current collector, and the coating film is dried to form a base layer. In an actual step, it is obtained in a state in which a current collector having a base layer of a base layer is formed on the surface thereof. In the coating of the base slurry, it is not particularly limited. As a coating method of the base slurry, a tumble coater, an air knife coater, a knife coater, a bar coater, a reverse coater, a bar coater, a comma coater, a tilt press coater, and a die coat can be used. Machine, gravure coater, micro gravure coater, screen coater, etc. In particular, the die coating method using a die coater is preferred in that the base layer is uniformly coated.

The drying method of the coating film is not particularly limited. In the drying of the coating film, in addition to natural drying, hot air, far infrared rays, microwaves, and the like can be utilized. However, it is generally dried in an oven that is integrally assembled at the rear of the coater. After drying, it is wound into a cylindrical shape in the winding portion of the coater. From rolling up to winding, it is preferably carried out continuously in a series of transfers.

In the current collecting system with the base layer of the present embodiment, the above-mentioned base layer is formed on the current collector. Hereinafter, the current collector will be described.

(collector)

The current collector is not particularly limited, and a current collector made of a conventional material such as aluminum, stainless steel, nickel plated steel, or steel can be used. In particular, in order to form the underlayer of the positive electrode, an aluminum foil is preferably used.

In the electrode system of the present embodiment, an electrode layer is further coated on the surface of the base layer of the current collector of the base layer. Hereinafter, the electrode layer will be described.

(electrode active material)

First, the electrode active material will be described. As the active material contained in the electrode layer applied to the surface of the underlayer, any active material generally used as the active material for the positive electrode and the active material for the negative electrode can be used. Regarding the underlayer of the present embodiment, since the effect is remarkably exhibited in the case of using the positive electrode, it is preferably used for the positive electrode. In the case where the underlayer of the present embodiment is used for an electrode, a lithium overplated metal composite oxide capable of occluding/releasing lithium ions can be used as an active material for a positive electrode, and examples thereof include lithium cobaltate and manganese. Lithium acid, lithium nickelate, and composite oxides (mixtures) thereof; lithium iron phosphate and the like. Among these, lithium manganate is preferably used in terms of electrode performance and cost.

(electrode adhesive)

Next, the electrode adhesive will be described. As the electrode adhesive, a chemically stable material such as polyvinylidene fluoride, polytetrafluoroethylene, EPDM, SBR, NBR or fluororubber is preferably used. In the case where an active material for a positive electrode is used, among these, polyvinylidene fluoride is particularly preferably used. Since polyvinylidene fluoride is dissolved in N-methyl-2-pyrrolidone, it can be used in the state of an adhesive solution, and is suitable for producing an electrode slurry.

(electrode conductive aid)

Next, the electrode conductive material will be described. As the electrode conductive auxiliary material, a carbon black material such as carbon black or natural graphite, artificial graphite or amorphous carbon, or a metal oxide such as titanium oxide or cerium oxide, or a metal fiber can be used. Among these, carbon black which exhibits a structure is also preferably used, and it is particularly preferable to use one of the furnace blacks or Ketjen black and acetylene black. Also, carbon black and other conductive agents, for example, can also be made into vapor grown carbon fibers. A mixture of (VGCF) is used.

(electrode slurry)

Next, the electrode slurry will be described. The electrode slurry can be obtained by adding the electrode active material, the electrode binder, and the electrode conductive auxiliary material as described above to a solvent and mixing. Further, the solvent also has a function as a dispersion medium for dispersing the conductive agent. The mixing ratio of these can be appropriately determined depending on the viscosity of the slurry, the strength of the electrode layer sought, or the adhesion force with the current collector. Further, for example, when polyvinylidene fluoride is used as an electrode adhesive, N-methyl-2-pyrrolidone is most suitable as a solvent.

(Production of electrode with base layer)

Next, a method of manufacturing the electrode will be described. An electrode having a base layer can be obtained by applying the above electrode paste and drying the coating film on the base layer of the current collector of the base layer. The coating method and drying method of the electrode slurry are not particularly limited, and any of those exemplified in the method for forming the underlayer may be employed.

In the lithium ion secondary battery of the present embodiment, an electrode having the above-mentioned underlayer is used. Hereinafter, a lithium ion secondary battery will be described.

(Lithium ion secondary battery)

First, a lithium ion secondary battery will be described. The lithium ion secondary battery of the present embodiment can be used by using the above-mentioned "electrode having a base layer" as one of the positive electrode and the negative electrode, and combining the counter electrode, the separator, and the electrolytic solution. By arranging the separator between the above-mentioned "electrode having a base layer" and the opposite electrode to make it face each other, The electrolyte is impregnated between the two electrodes including the separator to exhibit the function as a battery. The exterior decoration can use an aluminum laminate film or a coin-type case made of stainless steel.

(relative electrode)

Next, the counter electrode will be described. The counter electrode system is combined with the above-mentioned "electrode having a base layer" to form a pair of counter electrode groups, and those having opposite polarities are selected. In the case where the counter electrode is a negative electrode, the negative electrode active material to be used may be a metal material such as lithium or a metal material such as bismuth or tin; and a carbon material such as graphite or coke. A compound that occludes/releases lithium ions is used.

On the other hand, in the case where the counter electrode is a positive electrode, a lithium transition metal composite oxide capable of releasing lithium ions is exemplified as the positive electrode active material to be used. Examples thereof include lithium cobaltate, lithium manganate, lithium nickelate, and composite oxides or mixtures thereof; lithium iron phosphate and the like.

(electrolyte)

Next, the electrolyte solution will be described. Examples of the solvent for the electrolytic solution for the nonaqueous electrolyte secondary battery include a low-viscosity chain carbonate such as dimethyl carbonate or diethyl carbonate; ethyl carbonate, propyl carbonate, and carbonic acid High dielectric constant cyclic carbonate such as ester; γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate And any one of methyl propionate, vinyl carbonate, dimethylformamide, and cyclobutyl hydrazine, and a mixed solvent thereof.

Further, the electrolyte contained in the electrolytic solution is not particularly limited. Examples of the electrolyte contained in the electrolytic solution include any one of LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiCF ₄ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiI, LiAlCl ₄ or the like. a mixture of the above. It is preferably one of LiBF ₄ and LiPF ₆ or a mixture of two lithium salts.

(separator)

Next, the separator will be described. Examples of the separator for preventing contact between the positive electrode and the negative electrode include a polyolefin film made of polyethylene or polypropylene, a microporous film made of an aromatic polyamide resin, a non-woven fabric, and a porous material containing an inorganic ceramic powder. Resin coating, etc.

(assessment)

Next, an evaluation method of the lithium ion secondary battery will be described. Actually, the effect of the underlayer of the present embodiment can be measured by performing a charge and discharge cycle test after assembling a lithium ion secondary battery. The manufacturing method of the lithium ion secondary battery is not described in detail herein. In general, the coin battery type is easy to assemble and convenient, and is often used.

Hereinafter, embodiments of the invention will be described in detail.

[Example 1]

(Production of base slurry)

A pellet-shaped amorphous polyester resin having a bisphenol A skeleton in the main chain (a number average molecular weight: 22,000, trade name: Byron 290, manufactured by Toyobo Co., Ltd., and a Byron-registered trademark) was dissolved in MEK: toluene = The weight ratio of 1:4 was 1455 g of the dissolved solvent. After completely dissolved, 267 g of acetylene black was poured and stirred for 1 hour to be mixed. Thereafter, the diluted solvent was mixed in a weight ratio of MEK:toluene=1:1 to adjust the viscosity to obtain a base slurry.

(formation of the basal layer)

The base slurry obtained above is applied to the thickness by gravure coating On the 20 μm aluminum foil, the coating film was dried to obtain a base layer (collector with a base layer) formed on the aluminum foil. The thickness of the base layer was 4 μm.

(Production of electrodes)

A positive electrode slurry containing 100 parts by weight of lithium manganate as a positive electrode active material, 5 parts by weight of acetylene black as a conductive auxiliary material, and 4 parts by weight of PVdF (polyvinylidene fluoride) as an electrode binder as a solid component was applied by a die coating. The body (solvent N-methyl-2-pyrrolidone) was applied onto the base layer of the aluminum foil with the base layer obtained above, and the coating film was dried to form an electrode layer on the undercoat layer. At this time, the thickness of the electrode layer was 85 μm. The laminate of the electrode layer/base layer/aluminum foil obtained in this manner was used as a positive electrode.

The positive electrode obtained above was passed through a diameter of 13.5 mm and pressurized. The thickness of the electrode plate after pressurization was 79 μm.

(Production of battery (coin type half battery))

The electrode plate obtained above was used as a positive electrode, and a separator (polyolefin flat film) was sandwiched to face the metal lithium of the negative electrode to assemble a coin battery. An electrolyte injection solution obtained by dissolving LiPF ₆ in a solvent mixed with EC (ethyl carbonate): DEC (diethyl carbonate) = 3:7 by a concentration of 1 mol/L. A coin-type half-cell is fabricated in the interior of the assembled coin-type battery.

(Charge and discharge cycle evaluation test)

The coin-type half-cell fabricated above was placed in a charge and discharge evaluation device, and subjected to a cycle test at a charge and discharge rate of one cycle of charge and one cycle of discharge. The termination voltage is set to 4.25 V on the charging side and 3.00 V on the discharge side. Here, the one-cycle system is a current value at which the battery capacity is charged or discharged in one hour.

[Embodiment 2]

In the same manner as in Example 1, except that a scaly amorphous polyester resin having a bisphenol A skeleton in the main chain (a number average molecular weight of 14,000, trade name: Byron 296, manufactured by Toyobo Co., Ltd.) was used as the base adhesive. From the production of the base slurry to the charge and discharge cycle evaluation test.

Hereinafter, a comparative example will be described.

<Comparative Example 1>

(Production of electrodes)

A positive electrode slurry (solvent N-) containing 100 parts by weight of lithium manganate as a positive electrode active material, 5 parts by weight of acetylene black as a conductive auxiliary material, and 4 parts by weight of PVdF as an electrode binder as a solid component was applied by a die coating. Methyl-2-pyrrolidone) was applied onto an aluminum foil having a thickness of 20 μm without forming an undercoat layer, and the coating film was dried to obtain an electrode layer. The thickness of the electrode layer was 90 μm. The laminate of the electrode layer/aluminum foil obtained in this manner was used as a positive electrode.

The positive electrode obtained above was passed through a diameter of 13.5 mm and pressurized. The thickness of the electrode plate after pressurization was 78 μm.

Thereafter, from the production of the battery to the charge and discharge cycle evaluation test, in the same manner as in the first embodiment.

<Comparative Example 2>

(Production of electrodes)

A positive electrode slurry (solvent) containing 100 parts by weight of lithium manganate as a positive electrode active material, 5 parts by weight of acetylene black as a conductive auxiliary material, and 4 parts by weight of PVdF as an electrode binder as a solid component was applied by a die coating. N-methyl-2-pyrrolidone) is commercially available as a base adhesive having a thickness of 20 μm formed of a carbon-based layer formed of a scaly amorphous polyester resin having a bisphenol A skeleton in a main chain. The carbon coating of the product was applied to an aluminum foil, and the coating film was dried to obtain an electrode layer. The thickness of the electrode layer was 88 μm. The laminate of the electrode layer/aluminum foil obtained in this manner was used as a positive electrode.

The positive electrode obtained above was passed through a diameter of 13.5 mm and pressurized. The thickness of the electrode plate after pressurization was 79 μm.

Thereafter, from the production of the battery to the charge and discharge cycle evaluation test, in the same manner as in the first embodiment.

<Comparative Example 3>

An amorphous polyester resin (quantitative average molecular weight: 19,000, trade name: Byron 245, manufactured by Toyobo Co., Ltd.) having no bisphenol A skeleton in the main chain was immersed in a mixed solvent used for the electrolytic solution (EC: DEC = 3) After 7), confirm that it has swelled. Therefore, no subsequent evaluation is performed.

<Comparative Example 4>

An amorphous polyester resin having a bisphenol A skeleton in the main chain (a number average molecular weight of 11,000, trade name: Byron GK780, manufactured by Toyobo Co., Ltd.) was immersed in a mixed solvent used for an electrolytic solution (EC: DEC = 3) After 7), confirm that it has swelled. Therefore, no subsequent evaluation is performed.

Fig. 1 is a graph showing the results of the charge and discharge cycle evaluation tests performed in Examples 1 and 2 and Comparative Examples 1 and 2. 1st, Comparative Example 1 with an aluminum foil using an aluminum foil not formed with a base layer or a commercially available carbon Comparing with 2, it was found that Example 1 and Example 2 are superior in cycle characteristics (life characteristics). In particular, it has been found that although the initial performance is slightly deteriorated, the durability of performance is excellent in long-term life.

From the above results, it is understood that the amorphous polyester resin which is not dissolved or swelled in the mixed solvent used in the electrolytic solution is preferably an amorphous polyester resin having a bisphenol A skeleton and having a number average molecular weight of 14,000 or more. 22,000 or less.

[Industrial availability]

The present invention relates to a lithium ion secondary battery or the like for performing repeated charge and discharge.

Claims (9)

A base layer provided between a current collector and an electrode layer constituting an electrode of a secondary battery; comprising a conductive material and an adhesive agent for adhering the conductive material to each other in the main chain An amorphous polyester resin of a bisphenol A skeleton. The substrate of claim 1, wherein the electrically conductive material is a carbon material. The substrate of claim 2, wherein the carbon material is acetylene black. The undercoat layer according to any one of claims 1 to 3, wherein the amorphous polyester resin having a bisphenol A skeleton in the main chain has a number average molecular weight of from 14,000 to 22,000. A current collector with a base layer formed with a base layer according to any one of claims 1 to 4 on a current collector. The current collector of the base layer of claim 5, wherein the current collector is an aluminum foil. An electrode formed by laminating an underlying layer of a current collector of a substrate layer as claimed in claim 5 or 6. The electrode of claim 7, wherein the electrode layer contains an active material for the positive electrode. A lithium ion secondary battery comprising the electrode of claim 7 or 8.